Modern wind energy installations, in particular those in the relatively high power classes in the Megawatt range, are designed to operate at variable rotation speeds. This means that the rotation speed of the wind rotor can be matched to the respectively prevailing wind conditions by adjustment of the rotor blade pitch angle. While a low rotation speed is selected at low wind speeds, a high rotation speed is correspondingly selected at high wind speeds. With a constant torque in the rotor shaft between the wind rotor and the generator, this means that, the higher the rotation speed is, the higher the power that is transmitted and therefore also the yield of the wind energy installation. One difficulty is that specific maximum and minimum rotation speeds must be complied with because of limiting parameters in the wind energy installation. With regard to the rotor voltage of the generator, the restriction is that the voltage must not become higher than the maximum AC voltage which can be produced by the converter even on reaching the maximum (or minimum) rotation speed. Conventionally, the electrical step-up ratio of double-fed asynchronous machines is therefore chosen as appropriate. This makes it possible to ensure that the rotor voltage is appropriate for the converter limit values during operation. However, it has been found that the step-up ratio that is required per se is no longer feasible for very high-power generators. This is particularly true when wind energy installations are retrofitted. In order nevertheless to allow the high power to be transmitted, it is either necessary to replace the converter for one with a higher voltage limit, which is expensive, or to restrict the rotation speed range of the wind energy installation, which narrows the usefulness and therefore the yield of the wind energy installation. One particularly disturbing factor in restricting the rotation speed range, which is advantageous from cost viewpoints, is that the remaining rotation speed margin for wind strength fluctuations, in particular for gusts, is lost.
It is known that an undesirable rise in the rotor voltage when the network frequency is incorrect can be limited by switching to a different torque/rotation speed characteristic (US 2007/069522 A1). When the load is low, that is to say the rotation speed is below the synchronous rotation speed, the characteristic is shifted to a lower torque so that a new operating point is set at a somewhat higher rotation speed, that is to say closer to the synchronous rotation speed. The generator slip is thus reduced, thus reducing the rotor voltage. In a corresponding manner, when the load is high, that is to say the rotation speed is above the synchronous rotation speed, the characteristic is shifted to a higher torque, thus resulting in a new operating point being selected at a somewhat lower rotation speed. The slip and therefore the rotor voltage are therefore likewise reduced. This known approach has the disadvantage that the torque/rotation speed characteristic is shifted toward medium rotation speeds, thus inter alia reducing the intended maximum rotation speed. The gust margin is thus reduced and the torque load on the drive train also increases.